Peronospora destructor is a plant pathogen. It causes downy mildew on leaves of cultivated and wild Allium. Allium cepa (onion and shallot) is most often affected, while Allium schoenoprasum (chives) and Allium porrum (leek) are only occasionally affected. Downy mildew is a major disease of onion. The pathogen persists as mycelium systemically infecting onion bulbs, but is not known to be transmitted in onion seed. The pathogen can persist in the soil for several years as oospores. Systemically infected plants are dwarfed and pale green. Under moist conditions, the pathogen sporulates on the affected tissues and spreads to other plants, on the leaves and stalks of which it forms greyish-violet local lesions. Infected leaves are often entirely killed. Critical periods for infection have been determined.<ref name="Virányi">{{cite book | author = Virányi F | title = Downy mildew of onion. In The Downy Mildews (ed. DM Spencer), pp. 461-472 | publisher = Academic Press | year = 1981 }}</ref> Infected crops yield poorly, and produce distorted bulbs. Control is by crop rotation (at least 3 years between successive onion crops), use of healthy bulbs for planting (heat treatment has been used to eliminate the pathogen from bulbs), fungicide treatment of the bulbs for planting, and fungicide sprays of the foliage if downy mildew infection is nevertheless observed. SymptomsPeronospora destructor is an oomycete plant pathogen that affects the Allium genera, including A. cepa (onions), A. sativum (garlic), A. porrum (leeks), A. schoenoprasum (chives), and A. cepa var. aggregatum (shallots). P. destructor causes a wide range of symptoms in onions. From afar, downy mildew of onions can be detected by patches of yellowed plants incapable of growing to the size of surrounding healthy plants.Leaves: When looking at individual leaves, necrotic spots begin as yellowing spots that eventually turn brown or black as the leaf tissue dies. Older and outer leaves often show symptoms earlier than younger leaves. Leaf tips shrivel as the pathogen moves inward toward the stalk of the plant itself. The symptoms begin as elongated, pale yellow lesions which progress into small patches of fungal colonies that are gray in color. As the disease continues to progress, secondary infection by other pathogens may occur, leading to purple or brown colored spores in the lesions on the leaves, which characterizes the downy mildew disease.Stalks: The stalks of onion plants can also be infected by Peronospora destructor, with symptoms appearing as yellow or